The present invention generally relates to sensors and more particularly relates to tire sidewall torsion (SWT) sensors for a motor vehicle control system, with such sensors being mounted at varying distances from the tire rotational axis on the chassis or wheel suspension in a stationary manner and with such sensors interacting with at least one encoder mounted on or in the tire wall or with at least one conventional encoder mounted on or in the tire wall and exhibiting poles, wherein the output signals or output information of such sensors are/is transmitted to the motor vehicle control system after having been evaluated.
Many methods using tire sensors for detecting the forces and torque acting on the tires are known for controlling the driving performance of a motor vehicle. Tire sensors (SWT sensors) consist of one encoder mounted in or on the tire and at least one sensor (measuring sensor) that is mounted on the wheel suspension in a stationary manner and associated with the encoder. Whereas EP 04 441 09 B1 proposes to monitor the deformation of the range of the tire profilexe2x80x94the tire contact areaxe2x80x94, WO 96/10505 proposes to detect the deformation of the sidewall of a tirexe2x80x94the torsion deformationxe2x80x94by measuring a period of time that elapses between the passing of at least two markings arranged on the rotating wheel at a different radius in relation to the rotational axis. A tire sensor which detects a change in phase angle and/or the amplitude between the output signals emitted by the measuring sensors when the tire is deformed due to forces acting on the tire is described in WO 97/44673.
The object of the present invention is to create a device which can provide preprocessed or conditioned data records to the signal processing unit, so that the software of the signal processing unit can correct any errors in the raw data and determine the tire or wheel forces on the basis of the conditioned signal data with less calculating work. Then the signal processing can be arranged upstream or be part of the motor vehicle control system.
This object is solved by a generic device in that at least one analog and one digital signal conditioning and/or processing unit are provided between the motor vehicle control system and the sensors.
According to an advantageous example of an embodiment, the sensors provide at least twoxe2x80x94related to a change in phase anglexe2x80x94and onexe2x80x94related to a change in amplitudexe2x80x94evaluatable output signals in the form of essentially sinusoidal alternating current signals to the analog signal conditioning and/or processing.
For the purpose of determining the amplitude, at least one analog-to-digital converter is associated to the analog signal conditioning and/or processing.
Preferably, the device according to the present invention exhibits the following assemblies:
An analog signal conditioning with a transformer which converts the sinusoidal a.c. signal into a voltage;
a filter that suppresses signal interference;
an offset compensation unit that adapts the signal conditioning and processing unit to changes in the signal offset of the sensor;
a trigger circuit that converts the sinusoidal (analog) signal into a rectangular-pulse signal; and
a unit determining the amplitude, which detects the peak value of every half wave. Preferably, the unit determining the amplitude is characterized by
an isolation amplifier which separately amplifies the positive and negative half wave;
an inverter which reverses the negative half wave;
a switch triggered by the signal provided at the output of the trigger circuit, which switches the positive or negative half wave to the input of an amplitude value detector in correct phase sequence;
a register that retains the value of the amplitude; and
a circuit for deleting the register, which is controlled by the digital signal conditioning and processing unit.
According to a further embodiment of the device proposed by the invention, the digital signal conditioning has at least one logic with at least two period counters, and sequence control systems start or stop the counters for any zero crossovers with positive or negative edges of the signals of the first sensor arranged closer to the rotational axis and the counter content effective at the time is stored as a time stamp in the registers for any zero crossovers of the signals of the second sensor arranged further away from the rotational axis.
Preferably, at least one register is associated with the counters, and the content of the period counter is saved in the register for every zero crossover and then it is started again.
The device according to the present invention reduces the quantity of data of several inter-connected analog input signals in an advantageous manner, so that the information obtained can be processed further with few operations in a digital signal processing unit.
In the SWT sensor signal conditioning and processing electronics according to this invention, information is obtained on the basis of the changes in amplitude, period and phase relation of the input signals, and this information is used to calculate the transversal and longitudinal forces acting on the tires.
Moreover, the device comprises means to suppress undesired interference or to detect such interference and then to compensate it arithmetically during the subsequent data processing.
In this way it is possible to use only one signal processing unit, in particular only one DSP (digital signal processor) for processing the data records of all wheels of a motor vehicle.
The device according to this invention advantageously is a functioning real-time signal processing unit for a SWT sensor system.
It reduces the quantity of input data in a meaningful manner and automates. Thus, the load on the subsequent data processing is reduced and, hence, less requirements are posed with respect to the processor (lower software run times, lower pulse frequency, lower costs).
In addition, it allows deterministic further processing of the data that was generated.
The evaluation circuit can be extended and improved when the magnetic coding of the tire is not strictly uniform but exhibits varying periods in certain areas.
These deviations from the xe2x80x9cnormalxe2x80x9d period (or from the 50:50 pulse duty factor) would have to be so large that they can be definitely recognized as additional coding and not deformation.
In this way, the process of saving the pole pattern is made more safe, since the correct position of the tire can be definitely recognized by the characteristic pole pattern. Complicated software processes for pattern recognition would not be needed.
In addition, information on the type of tire (e.g. summer or winter tire) could be coded, and when these are used as additional information by the control software, they could lead to control algorithms that are better adapted.
The use of a DSP for the data processing is only one advantageous example of an embodiment. A microprocessor can be used in the same way.
The data word widths and pulse frequencies used in the figures and description show one possible realization. Future studies have to determine whether these requirements can be reduced while still maintaining sufficient accuracy.
The device according to the invention is an electronic system that, in a suitable manner, evaluates and preconditions the output signals of SWT (sidewall torsion) sensors to determine the transversal and/or longitudinal forces, so that the data obtained therefrom can be used for further processing in a system controlling the driving performance, in particular the driving stability.
The sensors used detect changes in distance and longitudinal deformation in tires or wheels of motor vehicles. They are sensors with a magnetoresistive bridge, which have an amplifier with a current output already arranged downstream in the sensor head. The output signal of these sensors is an alternating current with almost sinusoidal form, whose amplitude changes in relation to the tire distance and whose frequency changes in relation to the wheel speed. A direct current component is superimposed over the alternating current, so that the resulting total current can have only a positive sign (into the sensor).
Two such sensors are arranged on each wheel. Hence, the change in the chronological difference between the zero crossovers of the two sensor signals indicates the longitudinal deformation of the tire.
The encoders associated with the SWT sensors are arranged in or on the tire sidewalls of the wheels, and preferably a magnetizable powder is filled into them. The magnetization of the tire sidewalls consists of 48 pairs of poles N,S that are arranged radially and distributed equally along the circumference. Their assumed origin is in the tire or rotational axis.
Instead of the second SWT sensor, it would also be possible to evaluate the output signal of a conventional wheel speed sensor. Although this type of sensor provides no information about changes in distance, the longitudinal deformation of the tire can be determined as before by measuring the zero crossovers.
In order to be able to apply this sensor in accordance with the present invention, its pole wheel must have the same number of poles as the SWT encoders.